1. Field of the Invention
The present invention relates generally to techniques for reducing the effects of spoofers and repeaters in radio-frequency receivers and more specifically to methods and apparatus for enhancing the reception of global positioning system (GPS) signals in the presence of spoofers and repeaters signals.
2. Description of the Related Art
In recent years, multi-element antenna systems have proven themselves to be extremely effective at protecting GPS receivers from high power jamming. High dynamic range systems with digital back-end processing, such as those developed at Rockwell Collins, Inc., are capable of spatially nulling jamming signals by as much as 60 dB while simultaneously maintaining good antenna gain to desired signal sources.
Recently, however, there has been renewed interest in C/A code spoofers and repeaters (meaconers). A spoofer is a broadcast signal intended to look like a GPS satellite signal. The signal can fool the receiver into tracking it, but it will be unusable for navigation. This can result in either faulty navigation performance or the inability to achieve a navigation fix at all. The repeater case is slightly different in that the receiver itself may not ever be aware that anything is wrong. A repeater simply rebroadcasts the RF environment it receives, so the GPS receiver may derive a perfectly valid navigation solution. The problem is that the solution it derives is for the location of the repeater not the receiver itself. In some cases it may be obvious that something is wrong. If a coupled inertial system indicates a large velocity, for example, while the GPS solution indicates that the platform is stationary, a repeater threat may be suspected. Traditional spatial nulling systems are not always effective against spoofer and repeater threats because the signal levels may be too low to have any impact on the GPS signal-to-noise ratio (SNR). Spatial nulling systems are designed to optimize the SNR at their output without any special consideration to specific threat characteristics. The GPS signal itself will typically have an SNR of −30 dB, so a repeater or spoofer can be 20 dB or more above the GPS signal level without having any appreciable impact on the actual SNR of the GPS.
The present invention relates generally to techniques for mitigating spoofer and repeater threats to the GPS systems, and more specifically to methods and apparatus for detection and digital spatial nulling of spoofer and repeater threats to the GPS that would not be addressed by the baseline SNR maximization algorithms.
U.S. Pat. No. 6,421,000, issued to present applicant C. E. McDowell, entitled, “Method and Apparatus For Reducing Jamming By Beam Forming Using Navigational Data”, discloses a method of signal error reduction in position systems such as the Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), terrestrial or aircraft based pseudolytes, and the like, and more specifically to a method and apparatus for mitigating multipath signal distortion using a multi-element antenna array. The multi-element antenna array is used to discriminate between the directed signal and its multipath components based on spatial angle of arrival. A reference signal, provided by the positioning system receiver, is used to compute element weightings that are utilized to null out the multipath components before they reach the receiver.
U.S. Pat. No. 5,952,968, issued to present applicant C. E. McDowell, entitled, “Method and Apparatus For Reducing Jamming By Beam Forming Using Navigational Data,” discloses a method of reducing jamming in a global positioning system (GPS) satellite receiving system that includes the steps of: a) selecting an initial weight value corresponding to each antenna element; b) selecting a weight adjustment scheme for adjusting the weights; c) measuring a power output from the antenna array; d) obtaining navigational data representing the orientation of the array; e) calculating for a gain of the antenna array corresponding each of the GPS satellites, using the navigational data to provide an indication of the orientation of the array relative to each GPS satellite; f) estimating a power level of a received signal corresponding to each GPS satellite; g) solving for the signal to noise ratio for each GPS satellite, using the estimated power level corresponding to each GPS satellite; h) iteratively and continuously adjusting the weights to obtain a greatest value of the signal to noise ratio; and i) continuously repeating steps (c) through (h).
U.S. Pat. No. 6,933,885, issued to Stockmaster, et al., entitled, “Miniaturized Digital GPS Anti-jam for Space and Size Constrained Applications,” discloses a system for providing anti-jam protection that includes a two element antenna; a radio frequency downconverter providing an in phase and quadrature component of received signals by the two element antenna; and at least two analog to digital converters. The analog to digital converters are coupled to the radio frequency downconverter, wherein a first channel complex data and a second channel complex data formed from an output of each of the at least two analog to digital converters form a covariance matrix. A weight calculation is obtained from the covariance matrix providing optimal anti-jam suppression. The calculated weight for optimal anti-jam suppression is applied through hardware components.
U.S. Pat. No. 5,990,831, issued to present applicant C. E. McDowell, entitled, “FFT Implementation of Digital Antenna Array Processing In GNSS Receivers,” discloses global navigation satellite system (GNSS) receivers, including digital spatial nulling arrays, and a method of providing antenna pattern outputs using the same. The digital spatial nulling array receives multiple antenna element inputs. A fast Fourier transform (FFT) is applied to the multiple antenna element inputs to obtain frequency domain representations of the multiple antenna element inputs. The antenna pattern outputs are provided by the spatial nulling array as a function of the frequency domain representations of the multiple antenna element inputs.